Desk Research: Lubricated Plug Valves

“How exactly are lubricated plug valves lubricated?” This chance question prompted some furious pounding of the keyboards recently at the Valve World Americas offices, as we strove to fill the gaps in our knowledge. Fortunately, the noise soon abated, thanks to the richly detailed explanations available on many valve supplier websites. In case other readers might also benefit, please find below some of our many questions with (abridged) answers.

By KCI Editorial.

How Does a Lubricated Plug Valve Work?

A lubricated plug valve has a cavity in the center of the plug that runs down its axis. This cavity has a lubricant injection fitting at the top. A check valve prevents lubricant from escaping. Under pressure, the lubricant can flow out of the center cavity through radial passageways into lubricating grooves in the plug’s surface.¹ Lubricated plug valves incorporate small channels in the plug construction called radial holes that allow for the flow of a lubricant. The lubricant works along the length of the plug and on its seating surface. Due to the lubricant, the closing, opening, and throttling of a lubricated plug valve is made easier.²

What Does the Lubricant Do?

The lubricant has three functions: to protect the internal closing surfaces of the valve from corrosion, to seal the valve, and to contribute to low operating torque. With an eye to achieving the best possible action, it is therefore important to relubricate the valve.³

Lubrication prevents metal-to-metal contact of the valve sealing services by filming over bearing irregularities. No matter how finely ground a metal surface may be, the metal is a series of peaks and valleys.

As one metal slides against another, friction is set up and adhesion, shearing, or plowing may result. A protective film of sealant over the bearing area prevents metal-to-metal rubbing.⁴

How Often Should the Lubricant be Topped Up?

Generally, plug valve greasing should occur depending on the frequency of operation; a plug valve should be greased either after each job or after each five actuations. Some plug valves may need to be greased more often if they have higher internal operating pressures or other environmental concerns. Older valves may also need repeated greasing.⁵

What are Common Problems with Valve Greasing?

Different sealant options may have a different viscosity. Always read the packaging to make sure that you are filling to the proper PSI. Valves that do not seal properly may have other issues apart from needing to be greased, such as a leak that should be addressed. Failing to actuate at every 2,000 PSI may trap air bubbles within the valve, though actuating more often is not necessary. Over-greasing can be just as dangerous as under-greasing, leading to a high-pressure system and ultimately system failure.⁶

Are There Application Examples of Lubricated Plug Valves?

At a field operated by a Middle Eastern natural gas and petroleum company, API 602 gate valves in drain and gas blowdown lines experienced frequent failures. Following a casual conversation with a pipeline engineer, Flowserve suggested the field operator replace the failing gate valves with Serck Audco Super-H lubricated plug valves.

The design of plug valves prevents debris carried from a production well from blocking and closing the valve. The field operator purchased a small number of plug valves and installed them in a pilot project. in one test site, the Serck Audco Super-H plug valves outperformed the existing gate valves in terms of reliability. Sand and other well debris did not prevent the plug valves from closing. As a result, operational uptime improved immediately. The field operator then purchased plug valves to replace all the gate valves in sizes from 1 to 4 inches throughout the field and natural gas gathering facility.⁷